In order to make the measurement as accurate as possible, the researchers had to compensate for the air drag created by the atmosphere at low-earth orbit. This drag creates a tiny deceleration of the satellite, which would be sensed by the accelerometers as acceleration. Therefore, the researchers added an ion engine to the tail of the satellite that will emit ions at a rate that perfectly matches this deceleration.

GOCE is part of a larger ESA mission called the Living Planet Program, which will involve launching seven more satellites over the next two years, each designed to measure a different feature of the planet. For example, this summer, ESA plans to launch a satellite called SMOS to measure the earth's moisture and ocean salinity. Another satellite, called CrySat-2, will blast off at the end of the year to map ice coverage. In the past, both ESA and NASA have focused on launching larger satellites carrying many instruments. In 2002, ESA launched Envisat, a 10-instrument satellite, and NASA has an ongoing program called Landsat, which started in 1972 and is considered the gold standard for earth-science missions.

Wahr, who worked on the GRACE mission, says that the new mission is very exciting. "For those of us in the business, it is going to be wonderful," he says.